The invention relates to applying a protective coating against oxidation on thermostructural composite material parts containing carbon or some other material that is sensitive to oxidation at high temperature, such as boron nitride.
Thermostructural composite materials are characterized by their mechanical properties which make them suitable for constituting structural parts, and by their ability to conserve these mechanical properties at high temperature. They are constituted by fiber reinforcement densified with a matrix of refractory material which fills the pores in the fiber reinforcement, at least in part. The materials constituting the fiber reinforcement and the matrix are typically selected from carbon and ceramics. Examples of thermostructural composite materials are carbon/carbon (C/C) composites, and ceramic matrix composites (CMCs) such as carbon fiber reinforcement with a silicon carbide matrix (C/SiC) or carbon fiber reinforcement with a matrix comprising a mixture of carbon and silicon carbide (C/Cxe2x80x94SiC), or indeed a C/C composite silicided by being caused to react with Si (C/Cxe2x80x94SiCxe2x80x94Si).
Thermostructural composite materials very frequently contain carbon, whether constituting the fibers, constituting at least part of the matrix, or indeed constituting an interphase coat formed on the fibers to provide them with adequate bonding with the matrix. Thus, whenever such parts are used in an oxidizing atmosphere and at a temperature above 350xc2x0 C., protection against oxidation is essential in order to avoid rapid deterioration of parts made out of such composite materials. This also applies when boron nitride (BN) is used as an interphase component between ceramic fibers and matrix.
There exists abundant literature concerning the formation of anti-oxidation protective coatings for parts made at least in part out of carbon or out of graphite.
For thermostructural composite material parts containing carbon, and in C/C composite parts, it is known to form a protective coating at least in part out of a composition containing boron, and more particularly a composition having self-healing properties. A xe2x80x9cself-healingxe2x80x9d composition is a composition which, by passing to a viscous state at the temperature at which parts are used, can serve to plug any cracks which might form in the protective coating. Otherwise, in an oxidizing atmosphere, such cracks give the oxygen of the ambient medium access to reach the composite material and to infiltrate into the residual pores thereof. Self-healing compositions in widespread use are boron glasses, in particular borosilicate glasses. Reference can be made for example to document U.S. Pat. No. 4,613,522.
It is also known from document EP 0 609 160 to form a coating for protection against oxidation by means of a mixture of zirconium diboride ZrB2, colloidal silica SiO2, and silicon carbide SiC. It should be observed that in that document, it is recommended to avoid using titanium diboride TiB2.
The oxide B2O3 is the essential element in boron-containing protective compositions. It has a melting temperature which is relatively low (about 450xc2x0 C.) and it is good at wetting the carbon-containing surface to be protected. Nevertheless, when the temperature becomes higher than 1000xc2x0 C., B2O3 volatilizes and its ability to protect diminishes.
In addition, because its melting temperature is relatively low, the oxide B2O3 can be eliminated from the surfaces of parts by blowing from a flow of gas passing over said surface. Furthermore, B2O3 is hydrophilic and forms boron hydroxides which begin to volatilize at relatively low temperatures (from 150xc2x0 C.).
However, there exists a need to protect parts that are used in a moist environment at high temperature.
This applies in particular to the diverging portions of nozzles for hydrogen-and-oxygen rocket engines where the water vapor produced and ejected through the nozzle creates not only an environment that is moist and oxidizing, but also sweeps the surface of the inside wall of said diverging portion.
This also applies to C/C composite brake disks as used in aviation when landing and taxiing on wet runways.
Document EP 0 550 305 discloses a method of making a coating for protecting composite material parts that contain carbon in order to provide them with resistance against abrasion and against blowing. That method comprises forming a coating on the parts out of a mixture of a non-oxide ceramic powder (such as a carbide, nitride, boride, or silicide powder), a refractory oxide powder having healing properties by forming a glass (such as a powder of a silica-alumina mixture), and a binder constituted by a resin that is a ceramic precursor (e.g. a polycarbosilane, polytitanocarbosilane or similar, polysilazane, polyvinylsilane, or silicone resin), the precursor subsequently being transformed into ceramic. A protective coat is obtained with a non-oxide ceramic phase and a healing phase constituting two interpenetrating lattices, thereby offering the desired resistance both to abrasion and to blowing.
An object of the invention is to provide a method of providing protection against oxidation for a part made of composite material, which method provides a high degree of effectiveness, particularly in a moist environment.
This object is achieved by a method comprising: applying on the part a composition containing a mixture of at least one boride in powder form, at least one vitreous refractory oxide in powder form having healing properties by forming a glass, and a binder comprising a resin that is a precursor for a refractory ceramic; and curing the resin,
in which method, said boride powder is constituted for the most part by titanium diboride TiB2, and said powder of at least one vitreous refractory oxide comprises for the most part a borosilicate mixture.
The term xe2x80x9cborosilicate mixturexe2x80x9d or xe2x80x9cborosilicate systemxe2x80x9d is used herein to mean an association of boron oxide and of silicon oxide, i.e. a (B2O3, SiO2) system.
In addition to titanium diboride TiB2, the boride powder may include at least one other metal boride such as aluminum boride, e.g. AlB2 and/or AlB12, and/or silicon boride such as SiB4 and/or SiB6.
Surprisingly, and as can be seen from the examples given in the description below, such a composition provides effective and durable protection against oxidation, including in a moist atmosphere, and in spite of the presence of B2O3.
The binder can be constituted by a polymer that is a precursor for a ceramic selected from: polycarbosilanes, polytitanocarbosilanes, polysilazanes, polyvinylsilanes, and silicone resins. The polymer is preferably cured in air at a temperature below 400xc2x0 C.
Advantageously, a composition is applied to the part so that after curing it presents a thickness lying in the range 200 micrometers (xcexcm) to 700 xcexcm.
Also advantageously, the composition is applied to the part as a plurality of successive coats, with intermediate curing.
The ceramization (transformation) of the refractory ceramic precursor takes place at high temperature, ceramization can be performed after the composition has been applied and before first use of the part by heat treatment at a temperature which is typically higher than 600xc2x0 C., and in an inert atmosphere. Ceramization can also be performed at higher temperature in an oxidizing atmosphere, preferably at a temperature higher than or equal to 800xc2x0 C. Ceramization is then performed over a shorter duration, e.g. by flash oxidation in a furnace containing air, or by flame treatment in air, or by direct inductive coupling with a heating inductor when the nature and the shape of the part make that possible.
In a variant, ceramization can be performed directly during first use of the part when operating at high temperature.
When the part to be protected is made of C/C composite, the composition can be applied to the part directly, or after a refractory undercoat has been formed, e.g. made of SiC. Such an undercoat serves to form an additional barrier providing protection against oxidation, but it is subject to cracking. The undercoat can be formed reactively, e.g. using SiO gas, by chemical vapor deposition or infiltration, or by ceramizing a precursor, or it can be obtained by siliciding the C/C composite with silicon, giving rise to an undercoat of the SiCxe2x80x94Si type.
According to another feature of the method of the invention, it includes a prior step of impregnating the part with a composition containing at least one phosphate, for example aluminum or magnesium phosphate, impregnation being followed by heat treatment at a temperature higher than 600xc2x0 C.
In order to make it easier to apply, and in particular in order to adjust its viscosity, the composition preferably contains a solvent for the ceramic precursor resin. The composition can be applied by coating using a paint brush or a spray gun, and then eliminating the solvent by drying, and then curing the resin.
In order to increase the ability of the protective coating to withstand blowing, the composition can include additional fillers in the form of short fibers or xe2x80x9cwhiskersxe2x80x9d of refractory material, e.g. of ceramic material such as silicon carbide or alumina.
The invention also provides a composite material part containing carbon and provided with a protective coating as obtained by the method defined above. The part can be a C/C composite friction part or a diverging portion of a rocket engine nozzle.